Rigid multilayer formed sheet structures

ABSTRACT

THIS INVENTION PROVIDES ECONOMICAL MULTILAYER SHEETS THAT COMBINE HIGH BARRIER LAYERS WITH LAYERS OF RIGID POLYVINYL CHLORIDE OR IMPACT POLYSTYRENE.

United States Patent O 3,748,218 RIGID MULTILAYER FORMED SHEETSTRUCTURES Ritchey 0. Newman, in, Midland, and Walter J. Schrenk,

Bay City, Mich, assignors to The Dow Chemical Company, Midland, Mich.

No Drawing. Continuation-impart of application Ser. No. 8,058, Feb. 2,1970, now Patent No. 3,645,838. This application Oct. 4, 1971, Ser. No.186,291

Int. Cl. B32h 27/08 US. Cl. 161253 5 Claims ABSTRACT OF THE DISCLOSUREThis invention provides economical multilayer sheets that combine highbarrier layers with layers of rigid polyvinyl chloride or impactpolystyrene.

CROSS REFERENCE TO RELATED APPLICATION This application is acontinuation-in-part of application, Ser. No. 8,058, now US. Pat. No.3,645,838, filed Feb. 2, 1970.

BACKGROUND OF THE INVENTION Rigid formed containers are of many typesand sizes and are formed, filled and closed by a number of methods. Whenmade from sheet of any single polymer or copolymer they are limited tothe characteristics of that material.

SUMMARY OF THE INVENTION This invention provides multi-layer sheets formaking rigid formed containers. More particularly, the inventionprovides five-layer rigid sheets comprising an inner barrier layer,juxtaposed glue layers on both sides of the barrier layer and outerlayers of rigid polyvinyl chloride or impact polystyrene, said outerlayers having an ASTM D-790 flexural modulus of at least 150,000 psi.The sheet materials advantageously have a total thickness of 5 to 100mils, preferably about 7 to 80 mils.

Any unvulcanized natural or synthetic rubber which is soluble inmonomeric styrene, or can be rendered soluble in styrene, e.g. bymilling on compounding rolls, and contains at least one free olefiniclinkage per two basic units in the rubber polymer molecule, may be usedin making high impact styrene polymers which are useful in thisinvention. By basic unit is meant the radical making up the rubbermolecule, which in the case of natural rubber is the isoprene radical (CH having in its combined form one olefinic linkage per isoprene radical.In the case of synthetic GRS type rubber, the basic unit is the radicalcorresponding to the conjugated olefin, e.g. butadiene, chemicallycombined in the rubber molecule.

The outer layers can be from about 2.5 to about 40 mils thick and arepreferably from about 25 to 30 mils thick.

A wide variety of barrier materials may be employed in the central layerof films in accordance with the present invention. Particularly suitedas barrier layers are combinations of vinylidene chloride polymers,vinyl chloride polymers, vinylidene fluoride polymers and extrudablemixtures thereof. The requirement for the central layer is that thematerial be extrudable within a sheath, i.e. between layers, of anotherpolymer and that the composition have the desired gas and moisture vaportransmission barrier characteristics. Particularly advantageous andbeneficial are extrudable compositions of vinylidene chloride polymers,wherein the polymers contain at least about 70 weight percent vinylidenechloride, the remainder being one or more olefinically unsaturatedmonomers copolymerizable therewith. Suitable vinylidene chloridepolymers are prepared utilizing such comonomers as methyl, ethyl,

3,748,218 Patented July 24, 1973 isobutyl, butyl, octyl and 2-ethylhexylacrylates and methacrylate; phenyl methacrylate, cyclohexylmethacrylate, p-cyclohexylphenyl methacrylate, chloroethyl methacrylate,2-nitro-2-methylpropyl methacrylate, and the corresponding esters ofacrylic acid, methyl alpha-chloro-acrylate, octyl alpha-chloroacrylate,methyl isopropenyl ketone, acrylonitrile, methacrylonitrile, methylvinyl ketone, vinyl chloride, vinyl acetate, vinyl propionate, vinylchloroacetate, vinyl bromide, styrene, vinyl naphthalene, ethyl vinylether, N-vinyl phthalimide, N-vinyl succinimide, N-vinyl carbazole,isopropenyl acetate, acrylamide, methacrylamide or monoalkylsubstitution products thereof, phenyl vinyl ketone, diethyl furnarate,diethyl maleate, methylene diethyl malenate, dichlorovinylidenefluoride, dimethyl itaconate, diethyl itaconate, dibutyl itaconate,vinyl pyridine, maleic anhydride and allyl glycidyl ether. Commerciallyavailable light stabilizers may also be incorporated in the vinylidenechloride material such as tertiary-butyl salol. Other barriercompositions which may be used with benefit in films in accordance withthe present invention are vinyl chloride polymers which contain apredominant amount of vinyl chloride therein. Fluorocarbon polymers,fiuorohydrocarbon polymers and fluorohalohydrocarbon polymers may alsobe used with benefit. Such materials as polyvinyl chloride,polyvinylidene fluoride, chlorinated polyethylene and polymers of suchmaterials as vinylidene fluoride, vinylidene fluoride andchlorotrifiuoroethylene, chlorotrifluoroethylene and vinylidenefluoride, chlorotrifluoroethylene and vinyl chloride,chlorotrifluoroethylene-vinylidene fluoride and tetrafiuoroethylene andthe like can also be used. Generally, for economic reasons, thevinylidene chloride polymers are employed, as they are most readilyavailable at relatively low cost.

Beneficially, in the extrusion of the vinylidene polymers, it isfrequently advantageous and beneficial to incorporate therein a minorportion of a plasticizer, oftentimes a heat stabilizer and a lightstabilizer. Such additives are well known in the art and generally arefound advantageous in that the temperature required for the extrusion issubstantially reduced and the probability of decomposition of thepolymer in the extruder is lowered. Typical plasticizers which areemployed in the vinylidene or saran combinations are acetyl tributylcitrate, epoxidized soybean oil (commercially available under the tradedesignation of Paraplex G-60), dibutyl sebacate, and polymericplasticizers such as, for example, certain copolymers 'of ethylene andvinyl acetate.

The barrier layer can be from about 0.5 to about 35 mils in thicknessand is most advantageously from 1.5 to 20 mils thick.

The adhesive layer may vary in thickness from about 0.2 to about 5.0mils, however, generally the preferred adhesive layer thickness is about2.0 mils. Oftentimes the strength of the bond between the inner or corelayer and the outer or surface layer increases slowly as the thicknessof the adhesive layer is increased, and generally little or no increasein the bond strengths occurs after the thickness of the bonding layershas reached about 2.0 mils.

A wide variety of polymers and polymeric compositions are useful toincrease the adhesion between the rigid outer layers and the innerbarrier layer. Suitable polymers or polymeric compositions are readilyselected by determining the bonding strength of the composition beingevaluated by forming a two-layer extrusion. The bond strength of thetwo-layer laminate is readily determined by conventional peel strengthtests.

Adhesives which may be employed are copolymers of ethylene and vinylacetate, advantageously in proportions of from about 14 weight percentto about 40 weight percent vinyl acetate with from about 86 weightpercent to about 60 weight percent ethylene, copolymers of ethylene andethylacrylate advantageously in proportions of from about 20 to about 30weight percent ethylacrylate with from about 80 to about 70 weightpercent ethylene, copolymers of ethylene with isobutyl acrylate,advantageously in proportions of from about 20 to about 30 weightpercent isobutyl acrylate with from about 80 to about 70 weight percentof ethylene and chlorinated polyethylene, advantageously containing fromabout 15 to about 40 Weight percent chlorine. A particularly preferredadhesive is a block copolymer of styrene and butadiene. Blends of suchadhesive materials may also be employed.

The sheets of this invention are ideally suited for making containers ofa variety of forms for products that are sensitive to gases such asoxygen, carbon dioxide and the like or moisture, products that have anelusive fragrance that is desirably retained, products that may pick upunwanted odors from the environment or products that contain highlyvolatile Components. Products advantageously packaged in the sheetmaterial of this invention include, for example, cheese, butter,steroids, various medicines or drugs, dried foods and the like. Apreferred use of the sheet material is making drug blisters for theprotection of medicinal tablets from the deleterious effects ofmoisture, oxygen and carbon dioxide.

The invention is further illustrated by the following examples:

Example 1 A five layer film was prepared by the coextrusion of outerlayers of a medium impact polystyrene having a flexural modulus of about350,000 and comprising 3.5 weight percent of a copolymer of 76.5 weightpercent butadiene and 23.5 weight percent styrene having a solutionviscosity (5% in styrene at 25 C.) of 70icentipoises, 3.5 weight percentmineral oil, 0.2 weight percent di-t-butyl-p-cresol and the balancestyrene, a core layer comprising 96 parts by weight of a copolymer of 85weight percent vinylidene chloride and weight percent vinyl chloride, 3parts by weight of a copolymer of 67 weight percent ethylene and 33weight percent vinyl acetate having a melt index of 22-28 and a densityat 23 C. of 0.957 and 1 part by weight of tetrasodium pyrophosphate andglue layers comprising a styrenebutadiene block copolymer commerciallyavailable under the trade designation Kraton 2104. The outer layers hada thickness of 6.1 mils, the core layer had a thickness of 2.3 mils andthe glue layers were 1.6 mils thick.

Peel adhesion tests were carried out according to ASTM D-903-49 exceptthat a crosshead testing speed of 5.0 inches per minute was used. The180 peel test did not proceed satisfactorily because of the inability ofthe outer skin to be pulled at a 180 angle. Consequently, one outerlayer was physically separated from the remaining composite and wasinserted in the upper jaw of the Instron Tensile Tester with theremaining part of the composite structure being placed in the lowertester jaw. This allowed the unseparated portion of the specimen to liein a plane approximately 90 to the direction of pull. Peel tests werethen completed using a jaw span of 2.0 inches. The peel adhesion, inpounds per inch of width, was in the range of 2.8-4.1, average 3.5.While the initial separation was between the outer layer and the gluelayer, at times the separation would be between the glue layer and thecore layer, indicating that the adhesion in the two interfaces was aboutequal.

Example 2 The above experiment was repeated except that the glue layerscomprised a copolymer of 72 weight percent ethylene and 28 weightpercent vinyl acetate having a melt index of 6.0. In this example theouter layers were 4.0 mils thick, the core layer was 1,9 mils thick andthe glue layers were 0.9 mil thick. The peel adhesion in pounds per inchof width, was in the range of 0.7 to 0.8, average 0.77.

Example 3 The above experiments were repeated except that the core layerwas 1.60 mils thick and comprised 93.75 parts by weight of a copolymerof Weight percent vinylidene chloride and 15 weight percent vinylchloride, 4.50 parts by weight of acetyltributyl citrate, 1 part byweight of an epoxidized soybean oil commercially available under thetrade designation Paraplex G-60 and 0.75 part by weight of4-tertiarybutyl salol, the glue layers comprised the copolymer employedin Example 2 and were 0.35 mil thick and the outer layers each compriseda 2.60 mil thick layer of a propylene modified polyvinyl chloridecommercially available under the trade designation Airco 400 and havinga fiexural modulus of about 472,000. The moisture vapor transmissioncharacteristic for this material was 0.138 grarn/ 100 sq. in. in 24hours and the adhesion between layers was good.

Example 4 A five layer sheet is prepared by coextrusion as above withthe core barrier layer comprised of the copolymer employed in Example 2and glue layers comprised of the copolymer employed in Example 1. In thepresent example the core layer is about 1.6 mils thick and the gluelayers are each about 0.35 mil thick. The outer layers each comprise a2.6 mil thick layer of polyvinyl chloride having a flexural modulus ofabout 472,000 psi. The water vapor transmission rate for this materialis about 0.14 gram/ 100 sq. in. in 24 hours at atmospheric pressure andpercent relative humidity and the adhesion between layers is good.

Example 5 A five layer sheet is prepared by coextrusion as above withthe core barrier layer comprised of the copolymer employed in Example 2,outer layers each comprised of the polymer employed in Example 4 andglue layers comprised of a blend of about 50 weight percent of thecopolymer employed in Example 1 and about 50 weight percent of thecopolymer employed in Example 2. In the present example the core barrierlayer is about 2.0 mils thick, the glue layers are each about 0.5 milthick and the outer layers are each about 3.5 mils thick. The watervapor transmission rate for this material is about 0.09 gram/ sq. in. in24 hours at atmospheric pressure and 95 percent relative humidity andthe adhesion between layers is good.

It is conceivable that many other composite sheets having a Wide varietyof thickness and polymer or copolymer compositions would function wellfor diiferent end use applications. Thus, while certain representativeembodiments and details have been shown for the purpose of illustratingthe invention, it will be apparent to those skilled in the art thatvarious changes and modifications can be made therein without departingfrom the spirit and scope of the invention.

What is claimed is:

1. A rigid multilayer sheet comprising an inner barrier layer,juxtaposed glue layers on each side of said barrier layer and outerlayers, said outer layers comprising polyvinyl chloride having an ASTMD-790 flexural modulus of at least 150,000 p.s.i., said barrier layercomprising a. copolymer of vinylidene chloride and a remainder of one ormore olefinic-aly unsaturated monomers copolymerizable therewith, saidglue layers comprising a block copolymer of styrene and butadiene, saidrigid multilayer sheet adapted for thermoforming into a rigid containeror containers for packaging food products or drugs.

2. The rigid multilayer sheet of claim 1 wherein a copolymer of ethyleneand vinyl acetate is blended with said 5 block copolymer of styrene andbutadiene to form said glue layers.

3. The rigid multilayer sheet of claim 1 wherein a copolymer of ethyleneand ethylacrylate is blended with said block copolymer of styrene andbutadiene to form said glue layers.

4. The rigid multilayer sheet of claim 1 wherein a copolymer of ethyleneand isobutyl acrylate is blended with said block copolymer of styreneand butadiene to form said glue layers.

5. The rigid multilayer sheet of claim 1 wherein a polymer ofchlorinated polyethylene is blended with said block copolymer of styreneand butadiene to form said glue layers.

6 References Cited UNITED STATES PATENTS 3,673,050 6/1972 Newman et al.161-165 3,547,772 12/1970 Schnebelen 161-254 5 3,579,416 5/1971 Schrenk161254 3,630,826 12/1971 Rose et al. 161-190 GEORGE F. LESMES, PrimaryExaminer 10 W. R. DIXON, 1a., Assistant Examiner US. Cl. X.R. 161-164,242, 254, 255, 256; 260-876

